A
glance at the table reveals:
1.
The same compounds exist in fly ash and portland cement. Those of fly
ash are
amorphous (glassy) due to rapid cooling; those of cement are
crystalline,
formed by slower cooling.
2.
The major difference between fly ash and portland cement is the
relative
quantity of each of the different compounds. Portland cement is rich in
lime
(CaO) while fly ash is low. Fly ash is high in reactive silicates while
Portland cement has smaller amounts.
IS
3812 (Part 1): 2003 (Pulverized Fuel Ash —
Specification for use as Pozzolana
in Cement, Cement Mortar and Concrete) specifies Chemical requirements
for Fly
ash as tabulated below:
| Chemical
Requirements |
|
S.No
|
Characteristics
|
Requirements
|
| .
|
.
|
Siliceous
Pulverized Fuel Ash |
Calcareous
Pulverized Fuel Ash |
| 1
|
Silicon
dioxide (SiO2 ), plus aluminium oxide (Al2O3), plus iron oxide (Fe2O3)
percent by mass, min |
70
|
50
|
| 2
|
Silicon
dioxide (SiO2 ), percent by mass, min |
35
|
25
|
| 3
|
Reactive
silica in percent by mass, min |
20
|
20
|
| 4
|
Magnesium
oxide
(MgO), percent by mass, max |
5.0
|
5.0
|
| 5
|
Total
Sulphur
as Sulphur
tri oxide (SO3), percent by mass, max |
3.0
|
3.0
|
| 6
|
Available
alkalis as sodium oxide (Na2O), percent by mass,
Max |
1.5
|
1.5
|
| 7
|
Total
chlorides in percent by mass, Max |
0.05
|
0.05
|
| 8
|
Loss
on ignition, percent by mass, Max |
5.0
|
5.0
|
|
